Amplifying digital signal by ADA4870

Hi supporter,

I'm going to drive a series connected high power LED (http://www.ledengin.com/files/products/LZ4/LZ4-00DB00.pdf ) with a 30 to  50 MHz modulation frequency (square or sinusoidal wave)

aim to the output current capability and the voltage swing, i chose ADA4870 as my amplifier.

My issue is, the voltage source is a square wave that output from FPGA (LVTTL). If i want to amplify this digital signal by ADA4870, what should i notice on my design? or just set the configuration as the analog amplifier?

Any help would be appreciated.

Lin

  • Hello Lin,

    These blue LEDs are diodes and therefore you should drive them with current, not voltage.

    If you want to drive 4 of the diodes connected in series here is a circuit using the ADA4870 that will drive 700mA through the diodes. The 4 diodes in series is represented by the 1 ohm resistor in series with the 16.64V battery. The battery represents the voltage drop specified in the LED datasheet for 4 LEDs in series. This circuit needs another amplifier (not shown) because the source (V1) needs to sink 8mA. You can download teh ADA4870 SPICE model and copy the attached schematic if you want to investigate further. The bandwidth of this circuit is about 10 MHz so you will not be able to modulate @ 30MHz - 50 MHz.

    I don't believe you will be able to find an amplifier that can do 700mA peak current @ high supply voltage and @ 30MHz - 50 MHz signal frequencies.

    If you need 30 MHz - 50 MHz modulation, you should use the AD9665. This is an LED driver that is very fast (no problem for your frequency range) but you have to drive each LED with a separate AD9665 because the max voltage of the AD9665 is 5V. The AD9665 is easy to use. It requires only 4 resistors to set your output current but you would need one for each LED.

    Take care,

    Zoltan

  • Hi Zoltan,

    Thanks for your advice and nice simulation.

    I think i got some misunderstanding of the LED operation, thanks.

    I'll further try another circuit configuration to modulate the current signal.


    By the way,  thanks a lot for the information about AD9665, is there any evaluation kit of it? It will be good if the evaluation kit is available.

    Thanks for taking your time.

    Lin

  • Hi Lin,

    The 9665 eval board is in the works. We don't have them yet but please check back again in early June for availability.

    Zoltan

  • Hi Zoltan,

    I've read the AD9665's data sheet, please have a reference:

    http://www.analog.com/media/en/technical-documentation/data-sheets/AD9665.pdf

    http://www.analog.com/media/en/technical-documentation/data-sheets/AD9665.pdfAccording to the data sheet, i have something want to confirm and to ensure the understanding to the data sheet, please correct me if something wrong.

    1. The current signal which i want to amplified is sent input to WxSET, and the current signal of the input is V_WxSET / (200+4.32k), the formula is reference to Fig.14. So to my understanding, the controlling signal (FPGA, 30~50MHz LVDS signal) is sent into these pins which corresponding to the enable channel, is that right?

    2. This IC provides intrinsic oscillator controlled mode and extrinsic signal controlled mode by DIS/EN the OSCEN, is this IC have a low cut off frequency while using extrinsic controlled mode? because the frequency diagram of oscillator is recorded from 200 MHz, i'd like to confirm that it's ok if i used extrinsic controlled mode to drive around 50 MHz.

    3. What will be happened if the oscillator is enable and the WxSET also have a AC current signal input?

    4. What's the difference between the read channel and write channel? only different from the current gain? (Sorry that i'm new in this region)

    5. I'm making a footprint of AD9665, but the outline dimension of the top left (0.6/0.42/0.24) is different from the bottom left (0.5/0.4/0.3), is it true? or it's just a mistake?

    Thanks for taking your time.

    Lin